1. Field of the Invention
This invention relates generally to an electric jack stroke limit detection method and device for detecting the stroke limit of an electric motor-driven jack while the jack is being used to adjust the attitude of a mobile platform.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
There are a wide variety of commercial and industrial applications requiring mobile platforms that can be aligned relative to Earth's gravity (true level) by a known angle, or set of angles. The platforms are mobile and are often self-propelled, allowing them to be easily moved to various locations on the Earth's surface. However, once at a given location the platform must be supported and aligned relative to Earth's gravity before operating in its intended capacity. Examples of such platforms include: heavy industrial equipment, cranes, cherry pickers, and recreational vehicles.
The support and alignment of the platform is often accomplished through the use of jacks attached at different positions around the platform. The jacks may be extended to contact the ground, creating a rigid support base for the platform. By extending and retracting specific jacks, the platform may be aligned to at any angle allowed within the mechanical limits of the platform and jacks. The jacks may be hydraulically driven, or may be driven by DC electric motors.
With the advent of these platforms came the need for systems that can control jack movement (extension and retraction) and automate the task of bringing a platform to a known desired attitude. (Although, in the art, these systems are sometimes referred to as “mobile platform automatic positioning systems,” this document will refer to them as mobile platform automatic attitude adjustment systems, or just “platform attitude adjustment systems” for short. This is because the word “positioning” has connotations more closely related to translation of a body through space rather than the adjustment of the attitude of a body “in-place.” This document uses the word “system” to refer simultaneously to both a device and a process (or method) carried out by that device.)
Recent improvements in sensor technology, combined with the falling prices of semiconductors and microprocessors, are advancing the state-of-the-art in platform attitude adjustment systems. Where, in the past, jack movement was coordinated through the use of discrete circuitry and limited feedback, today it is known for computer processors to use new sensor technologies and advanced algorithms to adjust platform attitudes faster, safer, and more accurately than before. Today's mobile platform leveling or attitude adjustment systems are several orders of magnitude more sophisticated and powerful than their predecessors, allowing for unprecedented levels of control and reliability in their operation, but are configured to operate only hydraulically-actuated jacks.
It is beneficial for a mobile platform attitude adjustment or leveling system to include a control algorithm that takes into account jack position as well as remaining stroke length. A system using jacks to position a platform should, therefore, be able to detect when a jack has reached the maximum or minimum limits or ends of its stroke.
It is known for mobile platform leveling systems to employ DC electric jacks and for electronic controllers in such systems to detect when those jacks have reached maximum stroke limits while the jacks are being used to adjust the attitude of a mobile platform.
For example, U.S. Pat. No. 5,143,386 issued 1 Sep. 1992, to Uriarte (the Uriarte patent), discloses a mobile platform leveling system using DC electric motor-driven jacks and that is able to detect when any of those jacks reaches a maximum stroke limit. Specifically, the Uriarte patent includes a plurality of voltage comparator circuits that interface a controller to respective jack position status lines and a plurality of grounding switches, each connected in one of the voltage comparator circuits and positioned to ground that circuit when a corresponding jack is fully retracted. Without the costly addition of grounding switches, the Uriarte system would be unable to detect when jacks a jack has reached a stroke limit. The Uriarte patent also discloses current sense circuits that use Hall effect sensors sense the intensity of the magnetic flux due to the current flowing between each fuse and relay pair for each jack, which is in effect proportional to the current drawn by the motor on each jack 203. However, the controller doesn't use this proportional value to identify stroke limits.
Also, U.S. Pat. No. 4,084,830 issued 18 Apr. 1978, to Daniel, Jr. et al. (the Daniel patent), discloses a method for detecting when DC electric motor-driven jacks in a mobile platform leveling system have either fully retracted to respective inner stroke limits, or have extended to respective outer stroke limits. The Daniel patent discloses a controller connected to a plurality of upper limit switches that are supported in respective positions to detect when corresponding jacks are in respective fully retracted states. A plurality of lower limit switches are electrically connected to the controller and are supported in respective positions to detect when corresponding jacks are in respective fully extended states. The controller is programmed to interpret a signal from each of the upper limit switches as indicating full retraction of a corresponding jack and a signal from each of the lower limit switches as indicating full extension of a corresponding jack. Without the costly addition of the limit switches, the system disclosed in the Daniel patent would be unable to detect when a jack has reached a stroke limit.
What is needed is a method and device for detecting the stroke limit of an electric motor-driven jack without requiring the installation of jack position sensing devices and circuits such as grounding switches, comparator circuits, or limit switches.